Preparation of thin films of vanadium dioxide



Dec. 9, 1969 e. A. ROZGONYI PREPARATION OF THIN FILMS OF VANADIUM DIOXIDE Filed May 19, 1967 FIG.

CA THODE COMPOSITION AMORPHOUS FILM RESULT ,OF SPUTTER/NG WEAK OX/DA T/VEK/ S 7' E P STRONG 0x104 T/VE f STEP AMORPHOUS F [LM POLYCRKSTALL/NE F lLM POLYCRVSTALL/NE F/LM POLYCRYSIALL/NE FIG. 2

lNVENZ'OR G. A. ROZGONW IVEV United States Patent 3,483,110 PREPARATION OF THIN FILMS OF VANADIUM DIOXIDE George A. Rozgonyi, Irvington, N.J., assiguor to Bell Telephone Laboratories, Incorporated, Murray Hill, N.J., a corporation of New York Filed May 19, 1967, Ser. No. 639,902 Int. Cl. C23c 15/00 US. Cl. 204-192 3 Claims ABSTRACT OF THE DISCLOSURE A method has been found for making thin films of V0 that possess the essential metal-semiconductor phase transition exhibited by the single crystal forms, and that do not suffer deterioration under repeated cycling through the transition. The process involves the steps of sputtering with a V cathode to produce a film which is then oxidized to the desired oxidation state.

This invention relates to a process for making thin films of various vanadium oxides and to the products produced in accordance with the process.

BACKGROUND OF THE INVENTION Recent interest has been shown in several materials which are able to undergo a metal-semiconductor phase transition at a characteristic temperature. Accompanying the transition are abrupt and substantial changes in various properties of the material, such as changes in its electrical resistance, light reflectance, etc. Devices which make use of these changes have been devised. Exemplary of those devices which take advantage of the abrupt change in resistance are switching devices as described in US. 3,149,298, issued to E. T. Handelman. Other devices, such as optical modulators and display devices, utilize the change in reflectance that occurs as the material passes through its transition temperature to modulate or alter some characteristic of impinging light.

Among the materials which possess such a phase transition characteristic are various vanadium oxides, for example, vanadium dioxide (V0 and vanadium sesquioxide (V 0 It is important that these materials be in a form that is compatible with the modern planar device technology that is revolutionizing the electronics field. At present, however, these vanadium compounds are provided only as single crystals and not in the form of thin films for planar devices. Moreover, when single crystals of some phase transition materials are repeatedly cycled through the transition temperature, a fracture phenomenon often occurs causing a breakdown of the material, thus possibly imposing limits on the useful life of any device incorporating them.

SUMMARY OF THE INVENTION In accordance with the present invention a method has been found for making thin films of V0 and V 0 that possess the essential phase transition property exhibited by the single crystal forms, and that do not suffer deterioration under repeated cycling through the transition temperature.

In one form, the process involves the steps of sputtering a V 0 cathode in an inert atmosphere in the presence of a desired substrate to produce an amorphous film of a composition VO where x is greater than 1.5 but less than 2, and then either weakly oxidizing the film to end-product V0 or strongly oxidizing the film to V 0 and then reducing the V 0 so produced as a V 0 endproduct.

Alternatively, a vanadium cathode can be sputtered in an inert atmosphere in a similar manner to provide a r' evacuating the reaction chamber to 10* to l0 3,483,110 Patented Dec. 9, 1969 ice polycrystalline vanadium film which then is oxidized to V 0 and then reduced to V 0 Although the initial VO film is amorphous, the V0 V 0 and V 0 films produced in accordance with the inventive method are made polycrystalline during the 0xidative and/or reductive steps utilized.

DESCRIPTION OF DRAWING DETAILED DESCRIPTION As indicated by the flowsheet description of FIG. 1, the process of the invention can provide the vanadium oxides noted, VO V0 V 0 and V 0 Of course, thin films of V0 and V 03 are especially of interest because of their phase transition characteristic and resistance to recycling fracture.

Conventional cathodic sputtering apparatus may be used in carrying out the invention. By way of example only. the following briefly sets forth the system used.

The cylindrical cathode of either V 0 or V was 3 cm. in diameter and 3 cm. long. The vacuum system used was an all metal, sputter-ion pumped station with a watercooled cylindrical reaction chamber 10 cm. in diameter and 15 cm. long. The cathode-to-anode spacing was 2 to 3 cm., although other spacings would be acceptable. The pressures used were of the order of 10a to 300 although these can be extended to 1000p. with satisfactory results.

The effect of pressure on the sputtering operation is well known in the art. Increasing the pressure results in greater deposition rates due to the large number of bombarding ions present. However, at high pressures (relative to a perfect vacuum), the current flow obtained reaches undesirable limits. The lowest tolerable pressure is that which results in the smallest deposition rate that is economically acceptable. Operating pressures were established by torr and backfilling with argon or other inert gas or mixtures thereof to reach the desired operating pressure. The cathode density should be adjusted to within the range 0.1 to 10 ma./cm. the lower limit providing an adequate deposition rate, and the higher limit establishing a practical maximum to avoid short cathode life. Typical voltages to meet this requirement are from several hundred to a few thousand volts. Substrate temperatures may vary from C. to 500 C. with the quality of the deposited film improving somewhat at the higher temperatures. Preferably, greater temperatures are not used in order to avoid damage to the film or substrate.

The cathode material that is sputtered can form a thin film on a variety of substrates; by way of example, films of -VO and V have been successfully deposited on single crystal sapphire, amorphous glass, Si N and Ta O The thicknesses of the films produced ranged from 200 to 6000 A.

Substrate tempertatures are not critical, and temperatures within 100 to 500 C. are convenient. With a cathode of V 0 a thin film of amorphous vanadium oxide is produced which has a composition VO where x is greater than 1.5 but less than 2. It is theorized that the V 0 results in V O +O and that an amount of oxygen associates with the V 0 to provide a stoichiometric composition for the film between V 0 and V0 The VO film can be converted either to V0 or V 0 or V 0 by suitable post-deposition treatment involving oxidation and/ or reduction in open tube furnaces.

The just-described effect of oxygen on the stoichiometry of the film is, of course, more extensive when large amounts of oxygen are present during sputtering. It is found that if oxygen is present in an amount greater than about 50 percent (by volume), the VO composition noted is not reproducibly deposited. Accordingly, some precaution is taken to minimize this effect. Spectral grade inert gas is preferred, as is a procedure which bleeds gas from the sputtering chamber, from time to time, to remove oxygen produced during sputtering. In addition, baking the chamber at a few hundred degrees centigrade prior to backfilling with inert gas removes various impurities from the chamber walls that could possibly introduce unwanted gaseous species into the system.

If V is desired, the VO film is contacted with an oxidizing atmosphere of water vapor which is found not to strongly oxidize the film to an oxidation state higher than V0 Optimum results are obtained at a furnace temperature of 400-500 C. with contact maintained for about 4 hours. Other weak oxidants could also be employed to achieve a V0 composition.

On the other hand, V 0 is obtained with stronger oxidants, for instance by an oxygen-rich atmosphere. For this purpose a or greater O mixture proves excellent, at a furnace temperature optimally at 450550 C. for about 4 hours. The amount of oxygen is not critical to obtaining the desired end product, but secondary factors such as the rate of reaction depend on the oxygen concentration. The greater the oxygen content the more rapid the oxidation. The V 0 film produced is characteristically yellowish and polycrystalline.

Alternatively, V 0 is produced by sputtering a vanadium cathode to obtain a polycrystalline vanadium film that is typically black. The same sputtering conditions as described for V 0 are applicable. The same postdeposition treatment just described for producing V 0 from V0 is also operable on the vanadium film to It was not found possible to obtain V 0 films by direct treatment of VO or V films with oxidants alone. Apparently the tendency is for these lower states to reach a vanadium pentoxide equilibrium rather than an intermediate V O state. However, V 0 is obtainable from a V 0 film, regardless of whether it is produced by sputtering with a cathode of V 0 or V, by contacting it with a reducing atmosphere, such as wet hydrogen that has been saturated with water vapor (room temperature), while in a furnace at about 500-600 C. for about 1 hour. This reductive step takes considerably less time than the oxidative steps above described, indicating the relative ease with which the reduction step takes place (less active reductants would take longer). It was not possible, however, to reduce V 0 to the intermediate V0 FIG. 2 shows the resistivity vs. reciprocal temperature characteristics for the thin films of interest. The arrows indicate the direction of change.

Thin films of V 0 exhibit only the properties of a semiconductive material over the temperature range studied. The V0 and V 0 films, however, clearly show the phase transition from the semiconductor to the metal state which accompanies change in temperature through the transition temperature. The V0 shows a narrow hysteresis at about 65 C., which compares very favorably with the transition temperature for single crystal V0 Films of V 0 exhibited an abrupt transition to the metal state at about 110 C., with a transition back to the semiconductor state at about 145 C.

The resistance of these films to fracture was checked by cycling the V0 samples from room temperature to about 100 C., and by dipping the V 0 samples in and out of liquid nitrogen. The samples underwent repeated resistance changes of 10 ohms for V0 and 10 ohms for V 0 A point-by-point measurement after cycling did not reveal any change in performance.

The invention has been described with reference to particular embodiments and examples thereof, but it is intended that variations therefrom which basically rely on the teachings of the invention are to be considered as within the scope of the description and the appended claims.

What is claimed is:

1. A process for making a thin film vanadium oxide comprising the steps of sputtering with a cathode of a composition consisting essentially of V 0 onto a substrate heated to a temperature in the range from to 500 C., in an atmosphere consisting essentially of inert gas and up to 50 percent by volume oxygen at 10 to 1000 thus forming an amorphous film, and contacting said amorphous film with a weakly oxidizing atmosphere for a time sufiicient to form a polycrystalline V0 film.

2. The process of claim 1 wherein said amorphous film is contacted with water vapor at 400 to 500 C.

3. The polycrystalline V0 film produced in accordance with the process of claim 1.

References Cited UNITED STATES PATENTS 2,917,442 12/1959 Hanlet 204l92 3,294,669 12/1966 Theuerer 204298 OTHER REFERENCES Holland: Vacuum Deposition of Thin Films, Chapman & Hall Ltd., London, 1963, pp. 450-455.

ROBERT K. MIHALEK, Primary Examiner US. Cl. X.R. 

